In the related art, a centrifugal fluid machine (axial thrust reducing device of a centrifugal pump) described in PTL 1, for example, is used to solve a situation in which a strong axial thrust is generated in a direction of a pump suction port in an impeller due to the unbalance between pressure distributions in front and back of the impeller. This centrifugal fluid machine is a centrifugal fluid machine including an impeller obtained by boring a plurality of flow holes that pass through the inside of an impeller body radially from an impeller suction port that opens to a central portion on one side of a disk-shaped impeller body, and having an orifice portion of an annular gap formed between the impeller suction port and a pump case. In this centrifugal fluid machine, a pressure-equalizing hole penetrating in the axial direction without intersecting the flow holes is bored in the impeller body.
In the related art, for example, a centrifugal fluid machine (impeller of a centrifugal pump) described in PTL 2, is to improve suction performance without sacrificing pump performance. This centrifugal fluid machine is a centrifugal fluid machine in which an annular projection is provided on the back side of a rear shroud of an impeller to constitute a non-contact seal in conjunction with a casing, and a balance chamber is provided in a cavity inside the annular projection sandwiched between the rear shroud and the casing. In this centrifugal fluid machine, the balance chamber and a suction side of a impeller boss portion communicate with each other via a continuity hole provided in a center portion of the impeller, and a flow path that opens toward the downstream side on a conical surface on the suction side of the impeller.